In the printing industry, quality control of color printing processes typically has been achieved by measuring the optical density of a test image using a densitometer or scanning densitometer off-line of the web printing process. Optical density measurements are performed by illuminating a test image with a light source and measuring the intensity of the light reflected from the image. Optical density (D) is defined as: EQU D=-log.sub.10 (R)
where R is the reflectance, or ratio of reflected light intensity to incident light intensity.
The test image which is measured is often in the form of color test strips or color bars. These color bars are comprised of individual color patches of varying ink color and tone having dimensions approximately 0.2 inches by 0.2 inches, with the color patches laid out in a row adjacent one another. The color bars often extend across the width of the web, are printed in the trim area of the web, and may be utilized for registration as well as color monitoring purposes. As printers attempt to minimize paper costs through the use of "short-cutoff" presses (for example, the Harris-Heidelberg M-1000BE or M-3000 press), the trim area on the printed sheet becomes smaller, and accordingly, so do the color patches. The color patches presently may be as small as 0.10 inches by 0.0625 inches.
During the printing process, the color patches will occasionally be printed with imperfections. These imperfections include blemishes caused by a particle of foreign material caught in the ink train, spots caused by ink spray, dropouts of color caused by the blanket cylinder picking paper fiber and ink from the paper, and the overlap of two adjacent patches due to variance in registration. Another imperfection in all patches is caused by scatter within the paper. This causes the edges of a patch with white surrounding it to have a lower measured density than the rest of the patch, irrespective of the ink film thickness. This border effect may exist for approximately 100 microns at the edges of the patch.
In traditional automated color measurement devices, these imperfections will cause measurement errors. Unfortunately, heretofore there has been no reliable method to determine whether the patch has an imperfection. One approach has been to retain only the pixels within the patch having a color value which fits within a predetermined range, while excluding all other pixels having color values outside the range, labelling them invalid. The problems with this method derive from the setting of the predetermined range. If the range is too tight, there is a risk of rejecting legitimate color values. If the range is too wide, there is a risk of accepting color values associated with minor imperfections in the color patch.
One method for the optical density measurement of color patches in a color bar is disclosed in U.S. Pat. No. 5,724,259, issued Mar. 3, 1998. This patent also discloses a method and system for locating the individual color patches in an acquired image and correcting for the effects of scattered light.